Typical Sajaa main wellbore
30-in. casing at 70 ft 20-in. casing at 600 ft
Shuaiba Lateral 1 13 3⁄ 8-in. casing at 6,100 ft Kharaib Main wellbore 5-in. tubing Lekhwair Lateral 3 9 5⁄ 8-in. casing at 11,100 ft
> Coiled tubing drilling in the Middle East. BP-Sharjah initiated reentry CT drilling operations from existing wells of the Sajaa gas eld in the United Arab Emirates (left). The drilling con guration consisted of 23
8 Lateral 2
-in. coiled tubing and a 3-in. BHA with
a 4.1-in. bit. An in atable whipstock was set above the perforations to mill a window in the 7-in. casing of the main wellbore. Plans called for at least three horizontal sidetracks in each well (right).
7-in. liner at 14,400 ft
In April 2003, BP-Sharjah embarked on an underbalanced CT drilling program to perform through-tubing sidetracks from existing wells in the Sajaa gas-condensate fi eld, United Arab Emi- rates (UAE). The objective was to improve well productivity and unlock additional reserves with multilateral wellbores connected to existing primary vertical wellbores.
Plans called for setting flow-through, through-tubing whipstocks in 7-in. casing above existing perforations.
52
Since initial production in 1980, reservoir pressure in the Thamama limestone reservoir at 12,000 ft [3,658 m] true vertical depth (TVD) declined from 7,900 psi [54.5 MPa] to less than 2,000 psi [13.8 MPa]. Considerable gas and con- densate reserves remain, despite a significant 20% annual production decline in early 2003. The operator believed that overbalanced drilling had caused formation damage, resulting in extensive well cleanup. Recent horizontal rotary drilling programs had suffered massive, incurable lost circulation and severe differential sticking, which prevented some wells from reach- ing their geologic and drilling-length objectives.17 Underbalanced CT drilling operations were designed for wells previously completed with free-hanging 5-in. tubing inside vertical 7-in. casing.18
After milling a 3.8-in. casing-exit window, the CT drilling BHA— a specialized 3-in. wired CT drilling BHA attached to 23
⁄ 8 fluids, and -in. coiled tubing and
wireline heptacable, a PDM designed for com- pressible
either a 3.75-in.
polycrystalline diamond compact (PDC) or a 4.1- in. bicentered bit— would be used to drill underbalanced with nitrogen [N2] energized fl uids. Three or more openhole laterals were to be drilled to access up to 10,000 ft [3,048 m] of addi- tional reservoir per well (above). The initial phase of this campaign involved drilling 10 wells and 29 laterals with more than 66,000 ft [20,117 m] of new open hole. Up to fi ve laterals have been drilled from a single exit window. Threefold production increases are common. In several wells, underbalanced CT drilling has increased production from about 5 MMcf/D [143,200 m3/d] to more than 25 MMcf/D [716,000 m3/d], limited by the fl ow restriction of 5-in. production tubing.19
These successes moti-
vated BP-Sharjah to pursue additional CT drilling well candidates and extend the campaign. Schlumberger was recently awarded a two-year contract extension.
Directional hole sizes of 23
⁄4
-in. and 41
⁄8
-in. are
considered optimal for CT load capacities, hole- cleaning fl uid velocities and surface equipment specifi cations. However, 6-in. hole sizes and larger can be drilled under some conditions, particularly in vertical wells. Because of BHA limi- tations, directional CT drilling plans should target build rates less than 50° per 100 ft [30.5 m]. Exit- window depths and CT drilling lateral lengths should be evaluated on a case-by-case basis. Schlumberger is advancing CT drilling tech- nology worldwide through ongoing operations in Alaska, the Middle East, Venezuela and Indone- sia.20
Over the past fi ve years, average CT drilling lateral lengths have ranged from 1,500 to 3,000 ft [457 to 1,044 m]. With increasing activity, the CT drilling operating envelope continues to expand as evidenced by recent Schlumberger records:
• a 15,800 ft [4,816 m] whipstock casing exit in Colombia during 2002
• more than 9,000 ft [2,743 m] of open hole drilled underbalanced in a single reentry well in the UAE Sajaa gas fi eld during 2003
• the deepest whipstock casing exit at 16,240 ft [4,950 m] and deepest total CT drilling reentry depth of 17,515 ft [5,339 m] in Alaska during 2004.
Oilfi eld Review
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